Physical color is usually obtained either by diffraction of a surface grating, where first and higher diffraction orders such as in a hologram create color, as in U.S. Pat. No. 6,749,777, or by flat dielectric stacks such as interference filters. Under certain conditions strong colors are also obtained by combining very fine, sub-wavelength gratings, with one or several dielectric and/or metal stacks thereon. Such a zero order device (ZOD) uses zero order diffraction to produce very pronounced color effects.
Advantages of ZOD technology are: an iridescent optical effect, which varies with tilting angle Θ and/or rotation angle φ; optical effects easily recognized by untrained people; effects are machine readable; even under diffuse lighting conditions, strong color effects are achieved; this is a cost effective, mass producible technique; it is very difficult to counterfeit because the effect depends on the material properties. The deposition of high index material is on two different levels combined with an extremely fine, sub-wavelength grating. Copying only the grating or the material stack will not produce the desired effect.